Induction heating is a well known method for producing heat in a localized area on a susceptible metallic object. Induction heating involves applying an AC electric signal to a heating loop or coil placed near a specific location on or around the metallic object to be heated. The varying or alternating current in the loop creates a varying magnetic flux within the metal to be heated. Current is induced in the metal by the magnetic flux, thus heating it. Induction heating may be used for many different purposes including curing adhesives, hardening of metals, brazing, soldering, and other fabrication processes in which heat is a necessary or desirable agent.
The prior art is replete with electrical or electronic power supplies designed to be used in an induction heating system, many of which have inverter power supplies. Such inverter power supplies typically develop high frequency signals, generally in the kilohertz to megahertz range, for application to the work coil. Because there is generally a frequency at which heating is most efficient with respect to the work to be done, some prior art inverter power supplies operate at a frequency selected to optimize heating. Heat intensity is also dependent on the magnetic flux created, therefore some prior art induction heaters control the current provided to the heating coil, thereby attempting to control the heat produced. The prior art induction heaters described in U.S. Pat. Nos. 5,343,023 and 5,504,309 (assigned to the present assignee and hereby incorporated by reference) provide frequency control and a way to control the heat or power delivered to the workpiece.
Typical prior art induction heads are formed using rigid copper tubes. The tube is shaped to follow the contour of the workpiece or area of the workpiece to be heated. The rigidity of the tubing prohibits users from modifying the heat pattern (determined by the shape of the tubing) more than minimally. Consequently, induction heating customers have been required to purchase a unique (and expensive) coil for each heating application.
Also, copper tubing can be lossy at high frequencies. Approximately 90 percent the current is carried within two skin depths of the outer surface of a conductor (the skin depth of copper is about 0.0116 inches at 50 KHz, and decreases with increasing frequency.). Typical thin wall copper tube has walls approximately 0.046 inches thick. This is approximately four skin depths at 50,000 Hz. Thus, approximately 1/2 of the copper tube is not carrying any appreciable power. This forces the outer sleeve to carry more current than might be expected in such a conductor. Thus, the I.sup.2 R losses may be higher than desirable. The copper tubes were cooled using a coolant flowing through the middle of the tube.
One prior art induction head, described in U.S. Pat. No. 5,412,184, was used in an induction heating tool having a jaw like structure on which the induction coil was mounted. The jaw like structure is opened to receive a sleeve within the jaw. The jaw was then closed over the sleeve, and the sleeve was heated by the induction coil mounted on the jaw. The induction coil includes an inner coil for heating comprised of flexible copper bands surrounded by ferrite rods which help direct the flux to the part to be heated. Coils comprised of Litz wire on the outside of the ferrite (relative to the workpiece) reduce electromagnetic radiation from the tool, but do not heat the part. This tool suffers from several drawbacks. First, the applicability of the tool was limited to the distance which the jaws could open. Second, the coils, while flexible in a limited fashion, could only be shaped in a closed circle (in the jaws were closed) or a partially opened circle, when the tool is being loaded with a workpiece. The flexibility was also limited to a single direction because copper bands form the coil. Also, the tool was small and useful for heating only small workpieces.
There are at least two prior art arrangements used to inductively heat a large workpiece. One is to provide an induction coil shaped to generally coincide with the workpiece to be heated. This type of head suffers in that it must be dedicated to a single use. The other arrangement is to have a number of induction heads, each of which cures a selected portion of the workpiece. This type of head suffers that it is not useful for curved workpieces and requires multiple heads. Both of these arrangements are described in U.S. Pat. No. 4,950,348.
One new application of induction heating is using an induction heating system to preheat a thick workpiece, such as a pipe, before it is welded. Pipes are often formed by taking a flat piece of steel and rolling the steel. A longitudinal weld is then made along the ends of the rolled steel, thus forming a section of pipe. A pipe line may be formed by circumferential welding adjacent sections of pipe together. Other applications of welding relatively thick steel (or other material) include ship building, railroad yards, tanker trucks, or other higher strength alloy welding.
Pre-heating is used to raise the temperature of the workpiece along the weld path because the filler metal binds to the workpiece better when the weld path is heated, particularly when a high-alloy steel is being welded, and because it lessens undesirable fast cool-down. Without preheating there is a greater likelihood that the filler metal won't properly bind with the workpiece, and a crack may form. Preheating is often used for heating steel having a thickness of more than about 1/2", or when the part or ambient temperature is very low. (As used herein "preheating" a weld path means heating the workpiece along the weld path to a pre-weld temperature, or a temperature sufficient to reduce the likelihood of a crack forming along the weld path.) Generally, the steel is preheated to about 300.degree. F. prior to welding. Thus, when a weld is being preheated, the temperature can be high enough to damage nearby components. One system for using induction heat for preheating a weld is described in U.S. patent application Ser. No. 09/052,541, filed on Mar. 31, 1998, entitled Method and Apparatus for Welding, and owned by the present assignee. Prior to the use of induction heat, slow-heating gas fired "rosebud" heaters were used.
A similar pre-heating application is blades to pre-heat used on heavy machinery. Blades had been preheated in furnaces, and then moved to a weld location where a hard-coat was applied. Induction heat will work well for this application. Other heating applications for which induction heating could be used include curing/bonding painting, or infrared heating.
The workpiece temperature, and heating and cooling rates, also affect the grain structure of a weld. Generally, rapid heating and rapid cooling (heat stress) may adversely affect the grain structure of a weld, wherein the weld will be more prone to cracking. Accordingly, heat stress to a weld may be reduced by reheating a weld, or by controlling the rate of heating or cooling of the weld.
There would be a substantial advantage to a user if a flexible coil is used for induction heating: one coil could be used to heat different parts of various sizes and shapes. Such an application would be preheating of welding materials. Another application is preheating of motor windings prior to a trickle varnish coating, thus eliminating the need for vacuum impregnations.
Thus, an induction heating system with an induction head capable of being bent or formed to the shape of the workpiece to be heated is desirable. Preferably, the induction head will be capable of lying flat, or at least opening to 180 degrees, to allow it to be wrapped around the part to be welded. The induction head will preferably be capable of being wrapped around a pipe, so as to be capable of forming a complete circle. Additionally, such an induction head would preferably be capable of being formed to have sharp angle bends, so is to be adaptable for use with a wide variety of workpieces. Also, such an induction head will preferably include a thermal insulator to protect it from heat damage from the part being heated and to retain the heat in the workpiece.